Backflow preventers are used, for example, in water distribution lines to stop flow of possibly contaminated water back towards the potable water supply.
Conventionally, a backflow preventer has a unidirectional check valve and, downstream of the check valve, a relief valve. The check valve opens to permit flow through the line in the normal direction only, and closes to prevent backflow through the line, should the direction of flow reverse. The relief valve, closed when flow is normal, opens during a backflow condition to drain the possibly contaminated water from the line downstream of the check valve. This drainage through the relief valve is particularly important when the check valve malfunctions, for example, by failing to close completely because fouled by debris, because it helps to prevent water from flowing past the partially open check valve.
Backflow preventers, such as those shown in Braukmann, U.S. Pat. No. 3,818,929, and Griswald, U.S. Pat. No. 3,173,439, have relief valves which discharge through a single drainage passage.